1. Field of the Invention
The present invention relates to a high strength phosphorus-containing steel and a method for producing the same. In further detail, it relates to a carbon steel improved in strength by finely reducing ferritic grains, and to a method for producing the same.
2. Description of the Related Art
Conventionally, much effort has been made to remove P (phosphorus) in a smelting process for low carbon steel because P is known to have negative effects in the low temperature toughness of the product. However, the fact that the presence of P is not allowed had made it difficult to simplify the conventional smelting process, and the presence of P has been an obstacle in the reuse of steel materials.
Practically, for instance, in case P is contained at an amount of 0.1% by mass, the ductile/brittle transition point is increased by 40 K. Thus, carbon steel had been suffering the problem of embrittlement due to the presence of P, and in conventional smelting processes, great effort was necessary for the removal of P.
Apart from the problems above, the present inventors have been studying finely reducing the size of ferritic grains with an aim to develop high strength steel materials. In due course, it has been found that the transition temperature is considerably lowered by finely reducing the size of ferritic grains. Accordingly, it has been presumed that the problem of the embrittlement due to the presence of P can be overcome by finely reducing the size of crystal grains.
However, it was still a problem how to finely reduce the size of ferritic grains while controlling the presence of P.
The invention of the present application has been made in the light of the aforementioned circumstances, and it provides a high strength steel by overcoming the limits of the related art technology, and yet, by positively using the presence of P.